On The Strength Of New Composite Steelconcrete Material For Offshore Structure

Abstract

ABSTRACT The authors have developed a new composite steel-concrete material for the strength members of huge offshore structures, where concrete is placed between steel plates. This paper contains the results of both experimental and theoretical investigations into the strength of the new composite material. Experiments under static and repeated loadings were carried out. Four types of test models of the composite material were subjected to shear, bending or combined shear and bending. It was found that the ultimate strength of the composite material is very high and it can absorb a great deal of energy at failure. A method for the ultimate strength analysis of the composite material is developed. The calculated results are in good agreements with experimental ones. Nonlinear analysis is carried out using the finite element method. In the analysis, the material nonlinearities of both concrete and steel are fully taken into account; the propagation of a crack in concrete caused by tension, plastification and crush of concrete by compression, and plastification of steel plate. Geometrical nonlinearity caused by gaps between concrete and steel is, also considered. The analysis accurately represented the behavior of the composite material under tests. The new composite material is found to be desirable for the construction of offshore structures because of their excellent properties; large energy absorption and strength carrying capacity. INTRODUCTION The utilization of concrete and/or composite steel-concrete material for offshore structures gives the advantages of low initial cost, low maintenance cost and good impact resistance. Many projects using concrete are in various stages of design for offshore structures of near future such as an offshore power station. A huge offshore structure, if made of steel, necessitates a great deal of dead load to offset its big bouyancy, whether it is of the floating type or the gravity type. The concrete may be employed as ballast and strength members in such an offshore structure. Drawbacks of concrete associate with low tensile strength and low ductility. It is difficult to secure watertightness, once concrete cracking develops in an offshore structure used in deep water, where the pressure is very high. And this may cause serious troubles. If ordinary composite steel-concrete material, such as reinforced concrete or prestressed concrete, are employed, crack starts on the surface of concrete by bending. Drawbacks of concrete require a design with an excessive margin as the exact estimation of the external forces acting upon an offshore structure is nearly impossible. The authors have developed a new composite steel-concrete material for offshore structures to cover up the various drawbacks of the concrete. The new composite material endures large deformation and absorb a great deal of energy at failure, because the steel plate with high strength and ductility suppresses the development of surface cracks.